The present regenerator is usable in Stirling, pulse tube, Gifford-McMahon and Sibling Cycle cryocoolers.
Regenerative cryocoolers are required for a variety of applications in aircraft and spacecraft. These include linear Stirling cycle and linear drive Pulse Tube. Reliability and efficiency are critical considerations. Cost effectiveness is also important. Current regenerator technology for cryocoolers operating above about 50.degree. Kelvin (K.) is based upon stacks of screens woven from stainless steel wire. Packed lead spheres are commonly used for lower temperature.
Stacked screens have advantages and disadvantages. Much of the analysis of known devices and methods as set forth herein, including identifying advantages/disadvantages and their causes, is a part of the present invention and not prior art. Among the advantages are good heat transfer transverse to the fluid flow and poor heat transfer parallel to fluid flow. The disadvantages are several:
(1) Because heat transfer between fluid and mesh occurs mostly in the exposed portions of the wire between intersections of the wires, much of the surface area of the wire does not take part in heat transfer; PA1 (2) Because stainless steel has relatively poor heat conductivity, the thermal mass of the wire at the intersections does not participate usefully in the regenerative process. PA1 (3) Pressure drop through the regenerator depends upon the way in which the stacked screens match up with each other in the stack. It is possible for two screens to interlock in such a way as to seriously inhibit flow; variations in pressure drop of as much as 300% have been observed between ostensibly identical regenerators. PA1 (4) Because of the size and shape of flow passages through stacked wire screens, the ratio of pressure drop losses to heat transfer effectiveness is relatively high in stacked screen regenerators. PA1 (5) The cut ends of the wires in the screens are sharp, and it is impossible to completely immobilize the regenerator in its housing. As a result, the edges of the screen abrade the housing, creating debris that clogs passages and damages moving parts. PA1 (6) Stacked screens are made of very fine mesh wire cloth, which is expensive to weave and sensitive to clogging. PA1 (7) Before they are stacked, the screens must be cut with great precision and individually cleaned. PA1 (8) Because the screens are very thin, hundreds of screens must be stacked to achieve the necessary regenerator length, requiring a large quantity of wire cloth. PA1 (9) Assembly of hundreds of delicate screens in a regenerator housing is a tedious, time-consuming task for which no substitute for human labor has been found.
This invention relates to regenerative heat exchangers, specifically for cryocoolers, gas cycle heat engines, refrigerators and heat pumps.
At temperatures above about 50 K., stainless steel woven wire screen regenerators have been accepted as standard. However, regenerator theory indicates that the best geometrical configuration for a regenerator in terms of heat transfer and pressure drop is a parallel plate arrangement. (W. M. Kays and A. L. London, Compact Heat Exchangers, McGraw-Hill, New York, 1984; J. P. Holman, Heat Transfer, McGraw-Hill, New York, 1986; G. Walker, Cryocoolers, Plenum, N.Y., 1983). FIG. 3 from Radebaugh and Louie (R. Radebaugh and B. Louie, Proceedings of the Third Cryocooler Conference, NBS Special Publication 698, U.S. Government Printing Office, Washington, D.C. 1985, p. 177) shows that parallel plates with extremely small clearance are superior to stacked screen by a ratio of about 5 to 1 in terms of heat transfer. This prior work demonstrates that the highest heat transfer rate for a specified flow rate and pressure drop was thought to be developed with a parallel plate configuration.
U.S. Pat. No. 4,619,112 issued Oct. 28, 1986, discloses a spiral winding of a flat plate and a corrugated plate for a regenerator that seems to follow the above teachings by using the corrugations to obtain uniform channels of fluid flow with a large spacing between corrugations, because "the channel width uniformity is interrupted by the corrugations 1002 and deviations from channel width uniformity lowers the efficiency of the channel, the spacing between the corrugations must be large (e.g.) a factor of 5 to 6 or greater) in relation to height of the corrugations in order to maintain a high channel efficiency".
Although the theoretical superiority of parallel plate arrangements was known, regenerators continued to be built with stacked screens or packed spheres because nobody knew (W. Rawlins, K. D. Timmerhaus, R. Radebaugh, Measurement of the performance of spiral wound polyamide regenerator in pulse tube refrigerator, Advances in Cryogenics Engineering, Vol. 37, Plenum, N.Y. 1992, pp. 947-953) of a practical way to achieve a parallel plate regenerator. One major problem is heat conduction parallel to the fluid flow. Another problem is unevenly distributing flow among a series of channels between multiple parallel plates.
The concept of etching microchannels on a surface and capping them with a second, smooth surface, has been discussed in a number of publications. Pressure drop and heat transfer characteristics have been obtained for continuous (vs. alternating) flow in glass microchannels. (Peiui Wu and W. A. Little, Measurement of friction factors for the flow of gases in very fine channels used for microminiature refrigerators, Cryogenics, May 1983 pp. 273-277; Peiyi Wu and W. A. Little, Measurement of the heat transfer characteristics of gas flow in fine channel heat exchangers used for microminiature refrigerators, Cryogenics, August 1984, pp. 415-420).
Regenerative gas cycle machines are promising alternatives to a variety of successful technologies. They are currently used primarily as cryocoolers in low temperature refrigeration applications. However, gas cycle refrigerators show promise as replacements for CFC refrigerators currently in use for food preservation. Where the rejected heat is useful, gas cycle machines can be used as heat pumps. Gas cycle engines offer certain advantages over internal combustion engines and other types of heat engines.
In most kinds of service, regenerative gas cycle machines are competitive on efficiency grounds. However, the margin is small and only efficient, reliable, inexpensive gas cycle machines will be able to compete with other alternative systems. All of those gas cycle machines rely upon regenerators, which represent a major cost as well as a major source of inefficiency in conventionally-designed gas cycle machines.
The purpose of regenerators is to absorb heat while a fluid flows through the regenerator in one direction and release heat to the fluid when it flows through the regenerator in the opposite direction. Regenerators also act as obstructions to the flow of the fluids passing through them, and the resulting fluid friction reduces the efficiency of the machines in which they are employed. Design of regenerators to provide maximum heat transfer relative to fluid friction losses depends upon precise control of the internal geometry of the regenerator matrix.
In gas cycle machines, fluid is alternately compressed and expanded in a thermodynamic cycle. Compression ratio is an important factor in performance, and regenerators must be designed to provide the correct amount of void volume relative to volumes swept by pistons and displacers.
A traditional method of fabricating regenerators is to cut many layers of fine gauge metal wire cloth and stack those layers in a cylindrical housing to form a porous matrix. With stacked wire screens, regenerators are about 30% wire volume and 70% void volume with relatively minor variations from that relationship. Because it is impossible to control the exact position of successive layers of wire screens relative to each other, wire screen regenerators have a highly variable permeability, which makes it difficult to achieve reliable performance. Moreover, regenerators fabricated in this manner are expensive, partly due to the cost of the materials and partly due to the cost of cutting the screens and stacking them.
Beds of packed spheres are another possible alternative. Spheres of equal size pack to a density of about 60%, leaving 40% void volume between the spheres. While that method of regenerator construction avoids some of the expense of cutting and stacking screens, the spheres must be contained in some manner, usually by one or more layers of screen. In packed-sphere regenerators, heat conduction is approximately equal in all directions, which is not optimal. As with stacked screens, the ratio of solid volume to void volume is not adjustable beyond a relatively narrow range.
Other approaches to regenerator construction include felt-like materials fabricated from random wires of metal. These materials also have inherent variability that makes their geometry, and thus their performance, unpredictable. Small particles of wire may be created in the felting process; if dislodged into the stream of fluid passing through the regenerator, they can work their way into the fine clearances between pistons and cylinders, seriously damaging the machine.
Other methods of regenerator construction, such as metal and plastic foam, have been proposed. Foam materials suffer some of the same unpredictability of stacked screens and felt materials. They also have the potential to shed small particles into the fluid stream with deleterious consequences. Rolls of metal foil have been proposed as simple, inexpensive regenerators. By dimpling the surface of the foil slightly, it is possible to create rolls in which successive layers are separated from each other by the bumps in the surface, allowing a narrow passage for fluid flow between the layers. This approach suffers from at least two major drawbacks. First, the foil conducts heat well in the direction of the fluid flow but poorly transverse to the direction of fluid flow. That is the reverse of the desired relationship. Second, the foil blocks fluid flow in the direction transverse to the main fluid flow, making it impossible to adjust pressure differences between parallel layers. It is also difficult to mechanically emboss the foil surface in such a way as to create flow passages of accurate, uniform dimension.
U.S. Pat. No. 1,808,921 issued Jun. 19, 1931 discloses a plurality of sheets rolled into a heat exchanger coil core.
U.S. Pat. No. 4,619,112 issued Oct. 28, 1986 discloses a Stirling cycle machine with a coiled and corrugated foil core of a regenerator.